I got it working. I wanted a beam-break circuit which could detect a runner running down a narrow path, as in the attached picture. This is for a 100 mile foot race called the
Wasatch 100, which is happening this weekend. I want to know that a runner is about 1/2mi from our checkpoint, so we can get ready to welcome the runner.
I'm with the ham radio group that provides communications for the event. We team with other volunteers at one of the aid stations on the course. Our checkpoint is at mile 83, so the runners are quite spread out, and they arrive all night into the next day. I'm providing a travel trailer, antennas, radios, computers, packet modems, etc. for the event.
I hooked the beam break detector to a low-power VHF transmitter. The transmitter sends a coded message once each time the beam is broken.
The coded message opens the squelch on a scanner receiver at the checkpoint, giving us about 5 min warning that a runner is about to arrive.
To get reliable detection across the trail, I used
this IR detector for the receiver. I made an IR transmitter by building an oscillator that runs at 40kHz, and driving a Lite-On
LTE-5208A IR LED at 40mA at a 50% duty cycle.
The IR transmitter is left on continuously, aimed at the IR receiver. The IR receiver output is high with a steady signal, and goes low and then back high with a momentary interruption of the beam, such as passing a hand or body through the beam. The time constants in the receiver work just fine for a short interruption. I stretched the output pulse to key the radio transmitter.